1. Field of the Invention
The present invention relates to an antenna device, a wireless communication apparatus using the antenna device, a control method of controlling the wireless communication apparatus, a program product therefor, and a computer-readable storage medium therefor.
2. Description of Related Art
Recently, a wireless communication function has been often implemented in an information processing apparatus such as a personal computer, a communication terminal such as a mobile phone and a personal digital assistance (PDA), and any various kinds of consumer appliances such as an audio instrument, video equipment, a camera, a printer, and an entertainment robot. Further, such the wireless communication function has been often implemented in not only the electronics but also an access point for a wireless local area network (LAN) and a so-called accessory card of small size such as a card specified by personal computer memory card international association (PCMCIA), a compact flash card (trademark), and a mini peripheral component interconnection (PCI) card. The accessory card has been adapted to any wireless card module having such the wireless communication function and a storage function.
Under an actual application environment of these wireless communication functions, radio waves comes from various directions because there are any reflections by a building and an object or the like.
English Publication, “Small Beam-Switched Antenna with RF Switch for Wireless LAN”, by K. Mori. 34th European Microwave Conference, p. 837, on October 2004, discloses a Yagi antenna device of slot type, which can improve its communication performance by using a sector antenna (a directional antenna) This Yagi antenna device of slot type performs a communication test according to a WLAN communication system to increase gain of reception and/or transmission signals in the radio waves. Such the communication test allows a throughput to be relatively increased by compared with a related omnidirectional antenna.
Japanese Publication, “New Antenna Engineering” by Hiroyuki ARAI, Sougou Electronics Publisher, in 1996, discloses a Yagi antenna device as a typical directional antenna. FIG. 1 illustrates a configuration of a Yagi antenna device 10 of monopole type according to a related art. This Yagi antenna device 10 has a base disk 7 that is a grounding base, and a printed board 9 having antenna elements. The base disk 7 and the printed board 9 are combined with each other. The base disk 7 has an opening 6 through which a wire for power supply passes on a predetermined position thereof. The printed board 9 is positioned on the base disk 7 so that they are intersected with each other at right angles. The antenna elements are patterned on the printed board 9 with a parasitic antenna element 1 for waveguide, which has a length L1, an excited antenna element 2, which has a length L2, and a parasitic antenna element 3 for reflector, which has a length L3, being arranged in order (L3>L2>L1).
If radio wave having a wavelength of λ is radiated from the Yagi antenna device 10, the length L2 of the excited antenna element (monopole element) 2 is a quarter wavelength long. The parasitic antenna element 1 is away from the excited antenna element 2 by an optional distance D1. Similarly, the parasitic antenna element 3 is away from the excited antenna element 2 by an optional distance. The excited antenna element 2 is connected to a signal source 8 via a wired line extending from the excitation antenna element 2 to an end of the signal source 8 though the opening 6. The signal source 8 transmits a signal to the excited antenna element 2 through the wired line. The other end of the signal source 8 is grounded.
Thus, the Yagi antenna device radiates radio wave toward a direction like an arrow (directed from left side to right side of FIG. 1).
Japanese Publication, “Transactions of Institute of Electronics, Information and Communication Engineers” by MARUYAMA, UEHARA and KAGOSHIMA, Vol. J80-B No. 5, in 1997 discloses a multi directional Yagi antenna device. This multi directional Yagi antenna device has some Yagi antenna devices each similar to the Yagi antenna device shown in FIG. 1, which are directed toward some directions on a circumference of the base disk, so that the multi directional Yagi antenna device can get multiple directivities.
A phased array antenna and an adaptive array antenna are derived from the sector antenna. These array antennas reinforce the effective radiation pattern of the array antenna in a desired direction and suppress it in undesired directions, which is so-called as “beamforming”. These antennas can vary its directivity according to any receiving conditions of radio waves. Varying the directivity enables any communication performance to be increased. The communication performance is increased based on not only large gain of radio wave but also prevention of undesired radio wave from being received and transmitted.
Japanese Patent Application Publication No. 2001-24431 discloses the array antenna device relative to such the beamforming technology. This array antenna device constitutes electronically steerable parasitic array radiator (ESPAR). FIG. 2 illustrates an antenna device 80 with the beamforming functions. The antenna device 80 has a base disk 75, an excited antenna element 82, and parasitic antenna elements 81, 83, which are respectively arranged on both sides of the excited antenna element 82 at a suitable distance D2 from the excited antenna element 82. Items of variable reactance 84, 85 are respectively connected to the parasitic antenna elements 81, 83. If items of variable reactance 84, 85 are inductive, they act as extension coils, so that electrical length of each of the parasitic antenna elements 81, 83 can be extended to act as the reflectors. If items of variable reactance 84, 85 are capacitive, they act as shortened capacitor, so that electrical length of each of the parasitic antenna elements 81, 83 can be shortened to act as the waveguides. Thus, the antenna device 80 can radiate radio wave toward a desired direction by controlling the variable reactance 84, 85 of the parasitic antenna elements 81, 83.
In the above Yagi antenna device 10, if taking into consideration any performance on one-to-one communication by the wireless communication apparatuses, it is possible to improve any performance of throughput by using the directional antenna disclosed in the above English Publication, “Small Beam-Switched Antenna with RF Switch for Wireless LAN”.
In a wireless local area network (wireless LAN), user's wireless communication apparatus generally communicates with plural access points ordinarily under the circumstances of home, office or the like. The user's wireless communication apparatus and the plural access points constitute a network. On the network, frequency bands and channels, which can be used by the plural wireless communication apparatuses, are fixed and finite according to their capacities. If any control is performed on them, any collisions and/or interferences of the radio waves occur between the plural wireless communication apparatuses, thereby causing only the incomplete communication to be implemented.
In a standard 802.11 on the wireless LAN, an access control function is installed in order to avoid the collisions and/or interferences of the radio waves. This standard is called as “carrier sense multiple access with collision avoidance (CSMA/CA)”. According to the standard CSMA/CA, when a user wants to communicate with any destination, it is first sensed whether any other than the wireless communication apparatus that communicates does not communicate. The wireless communication apparatus can communicate only if it does not interfere with this other wireless communication apparatus (see Japanese Publication, “Realization of high-speed communication and its stabilization, the newest antenna technology, MIMO, WIRELSS PLUS”, by Eiji TAKAGI, Web Magazine, in 2004).